For many applications, polymeric products have been fabricated into structural or block forms, such as fibers, which utilize properties based on their high molecular weights. In recent years, applications have been developed, such as plates, sheets, films and the like, in which clarity or transparency is a very desirable property. Clarity is also very important for certain plastic articles, such as syringes, made by injection molding.
In general, clarity is not an inherent property of polyolefin plastics, most of which are more or less opaque due principally to their partially amorphous nature. Most polyolefins do, however, have some crystallinity, and they are generally referred to as semicrystalline. High clarity is thought to be related crystal size. Large crystals reduce clarity, an effect generally thought to be due to diffraction and scattering of light, and most polyolefins of good clarity are predominantly microcrystalline.
Various additives for polyolefins, conventionally referred to as nucleators, which promote crystallization at many sites, have been disclosed. U.S. Pat. No. 4,016,118 to Hamada et al. teaches improved clarity and resistance to mold shrinkage in polyolefins containing from about 0.1% to 0.7% of dibenzylidene sorbitol. Other dibenzylidene sorbitol clarifying additives postulated to function by reducing crystal size are disclosed in U.S. Pat. No. 4,371,645 to Mahaffey and U.S. Pat. No. 4,483,952 to Uchiyama. In the former patent, the phenyl rings are substituted with at least one halogen atom, and may additionally have alkyl, hydroxy, methoxy, amino or nitro substituent groups. The dibenzylidene sorbitols of the latter patent have chloro, alkyl or alkoxy substituents in each of the phenyl rings.
Prior art dibenzylidene sorbitol additives have resulted in polyolefin composition having improved clarity and resistance to shrinkage without loss of mechanical strength. There is, however, a definite need for further improvements, particularly with respect to molded articles which may be subject to subsequent sterilization procedures, such as, for example, heat or radiation, which may lead to discoloration and embrittlement due to oxidative deterioration. It is toward the fulfillment of this need that the present invention is directed.